1. Field of the Invention
The invention pertains to surface wave devices and reflecting mechanisms used with such devices.
2. Description of the Prior Art
In the construction of surface acoustic wave devices it is often desired to provide a reflecting mechanism for directing surface waves between input and output transducers or along a predetermined path so that various operations may be performed upon the propagating surface waves. Probably the most common reflecting mechanism thus far used is the reflecting grating. This has heretobefore consisted of either a number of parallel grooves cut into the surface of the substrate upon which the surface waves propagate, or else a number of parallel conductive stripes positioned upon the surface of the substrate. The reflecting grooves or stripes are ordinarily positioned so that reflections from adjacent ones of the grooves or stripes will be additive in phase so that the grating will be maximumly reflective. Unfortunately, only a very small percentage of the energy contained in an incoming surface wave would be reflected by any one groove or stripe within the grating. Total overall reflectivity of the grating could be improved by adding more and more reflecting elements. However, the bandwidth of the reflecting grating generally decreased in inverse proportion to the number of reflecting elements provided. Thus, with a high reflectivity grating, only very small bandwidths were typically available. In many devices such as filters, it has been desirable to to provide both high reflectivity and broad bandwidth. With the reflecting grating of the prior art it was frequently not possible to construct a surface wave filter device having both desired properties. Using a smaller number of reflective elements within each grating to obtain a required bandwidth often made the insertion loss of the device higher than desired because of the loss of signal at the reflecting gratings.
With reflecting gratings comprises of metal stripes, the reflectivity per element due to piezoelectric shorting is fixed, depending upon the type of piezoelectric material used for the substrate. Gratings constructed using metal strips provided periodic mass loading as well as a topographical perturbation and hence die have a reflection coefficient which could be increased by increasing the thickness of the metal. However, nonlinear effects upon the waves and conversion of surface wave energy to bulk wave energy became evident as the thickness was increased beyond certain limits.